The systems heretofore used in the art for measuring the flow of water or waste water in a partially filled pipe or other channel have typically relied upon combinations of pressure cells and velocity sensors which are individually installed within the channel. In order to determine the depth of the flow in the partially filled channel, the pressure cell is mounted on the bottom of the channel to measure the amount of hydrostatic pressure head produced by the flowing fluid. Once the depth of fluid flow is known, the cross-sectional area of the flowing stream can be determined based upon the known cross-sectional shape and dimensions of the channel. The rate of fluid flow is then calculated by multiplying the cross-sectional area of the fluid flow by the flow velocity.
Unfortunately, the pressure cell systems heretofore used in the art have significant shortcomings. The accuracy of the pressure cell system depends upon the pressure cell being mounted precisely on the bottom of the channel and upon the pressure cell and the velocity sensor(s) being in precise vertical and horizontal alignment. Precisely mounting and aligning the pressure cell system components in the field is difficult and time consuming. In addition, considerable time and effort is required when using the prior art pressure cell systems for field calibration, validation, programming, and channel profiling. Moreover, the pressure cells used in these systems are highly susceptible to fouling and sediment build-up which affects the accuracy of the sensor and eventually renders the sensor inoperable. Consequently, the pressure cell must be frequently removed and cleaned.
One type of velocity sensor preferred for use in partially filled channels is a chordal, transit-time sensor comprising a pair of piezoelectric elements which are mounted on opposite sides of the channel. The elements must be beneath the fluid flow for operation and are typically mounted at a height of at least one-quarter, more preferably about one-third, of the vertical inside diameter of the channel. Transit-time velocity sensors are effective for determining the chordal velocity of the fluid across the entire path of the fluid flow. In addition, the sensor elements are highly resistant to fouling and are not susceptible to drift problems, variable water surface problems, or non-uniform particle distribution problems which are frequently encountered with other systems. The transit-time velocity sensor elements are also designed to prevent accumulation of rags, branches, and other debris.
Unfortunately, as indicated above, it has heretofore been difficult and time consuming to mount and precisely align the velocity sensor elements with each other and with the pressure cell for proper operation and accuracy.
As an alternative to the use of transit-time velocity sensors, trapezoidal flume structures have sometimes been used for measuring fluid flow in partially filled channels. Based upon the depth of flow through the trapezoidal flume, the rate of fluid flow can readily be determined using known flume equations recognized by the U.S. Bureau of Reclamation. Trapezoidal flumes are generally effective over a broad range and have a flat bottom design which is generally effective for preventing the accumulation of sediment or debris.
Although trapezoidal flume structures are resistant to fouling and sediment accumulation, the proper installation and alignment of these systems in the field is difficult and time consuming and typically requires that the operation of the water or waste water system in question be disrupted for a considerable period of time.
It is also known in the art that an ultrasonic transducer installed in the top of a partially filled pipe or other channel can be used to determine and monitor the fluid level in the channel by transmitting an ultrasonic signal directly downward onto the fluid surface. However, in addition to being difficult to properly install and align, the ultrasonic sensor systems heretofore known in the art have not been capable of operation, or have only been capable of limited operation, in many channel systems. Ultrasonic sensors typically require a minimum signal transmission distance of about 12 inches or more. Thus, in order to operate an ultrasonic level sensor in a partially filled channel, it has heretofore been necessary for the liquid level in the partially filled channel to remain at least one foot below the ultrasonic transducer. These systems therefore have generally not been acceptable for use, for example, in pipes having diameters of less than about 24 inches or in applications where the liquid level will sometimes rise to within at least one foot of the top of the channel.
Thus, a need currently exists for a flow measurement system for partially filled channels which is accurate over a broad range of flows and is not susceptible to fouling or sediment buildup. A need particularly exists for a system of this type which can be quickly and easily installed in existing or new channel systems while ensuring that all of the system components are properly and precisely oriented and aligned. There is also a need for a system for measuring flows in partially filled channels which can be installed and activated substantially without the need for field calibration, flow profiling, and field programming and which does not require a significant amount of field validation.